The present invention relates to a multilevel recording and reproduction method and a phase change multilevel recording medium.
As the volume of information increases in recent years, there are growing demands for recording media capable of reading (reproducing) and writing a large amount of data with high density and at high speed. Optical recording media, particularly optical discs, are expected to meet such demands. The optical discs are available in two different types: a write-once type that allows the user to record data only once, and a rewritable type that allows the user to record and erase data as many times as they wish. Examples of the rewritable optical disc include a magnetooptical medium that utilizes a magneto-optical effect and a phase-change medium that utilizes a change in reflected light intensity accompanying a reversible crystalline state change.
The phase change medium can be written and erased by simply modulating the power of a laser beam without requiring an external magnetic field and thus has the advantage of being able to reduce the size of a recording and reproducing apparatus. It is also possible to enhance the recording density by using a light source with a shorter wavelength without specially changing the material of a recording layer of media which are currently recorded and erased with a light source with a commonly used wavelength of about 800 nm.
In the currently available rewritable phase change recording media, the crystalline state is taken as an unrecorded/erased state, and amorphous mark is formed. The amorphous mark is formed typically by heating the recording layer to a temperature higher than the melting point and quickly cooling it. Erasure (crystallization) is done by heating the recording layer to a temperature higher than the crystallization temperature of the recording layer, but lower than a temperature just above the melting point or the melting point itself. In a so-called one-beam overwritable phase change medium, the erasure and re-recording processes can be performed only by modulating the intensity of one focused light beam. In the 1-beam overwritable phase change medium, the layer configuration of the recording medium and the circuit configuration of the drive become simple Hence, this medium draws attention as a possible medium for use in an inexpensive, high-density, large-capacity recording system.
As described above, the phase change medium can increase the recording density by shortening the wavelength of a focused light beam to reduce its diameter and therefore the size of recorded marks. At present, laser diodes with a wavelength of 780 nm and an output of about 50 mW are widely available at low prices and applied to a phase change recording technology for rewritable compact discs, for example. Laser diodes with 630-660 nm are also available recently and a rewritable DVD is nearing the practical use along with the development of a high-output red laser diodes with an output of about 30 mW. With the demands for a higher density continuing, attempts to realize the recording density about two to three times that of the DVD by using a blue laser (about 400 nm) diode are actively under way though at a very preliminary stage of development.
There is naturally a limit to the recording density, however, if an increase in density of the phase change medium depends simply on the shortening of the wavelength of the light source. There are many problems to be solved as to the longevities of the laser diodes with short wavelengths and high outputs, and it will take time before such high-output laser diodes, though experimentally successful, can be put to practical use. Further, as the spot becomes smaller in size, problems arise, for example, an increased influence of tilt of the surface of a focused point and a reduced focus offset margin due to a shallower focal depth. Another question whether the amorphous marks, when they become smaller than 0.01 xcexcm, can remain stable, has not been solved yet.
An effort to increase the recording density of a magnetooptical recording medium dependent solely on the miniaturization of the read/write beams will naturally encounter a limitation due to the optical resolution capability (limit). In a phase change medium in particular, a so-called magnetic super-resolution phenomenon cannot be expected. Although there are some proposals on a super-resolution phenomenon utilizing a change in refractive index due to temperature changes, this method has an intrinsic problem that the recorded marks will deteriorate over repeated reading operations.
Spotlighted as one of the methods that transcend the limitation of the optical resolution capability (limit) and allow for an increased density beyond the optical resolution limit is a multilevel recording. This is a technology for a read-only compact disc which, rather than modulating the mark length, controls the depths of pits in a substrate in multiple levels to express the modulation in multiple values (xe2x80x9c15GB and No Blue Laserxe2x80x9d, Data Storage, April 1994 issue, cover story and pp27-32).
Such a multilevel recording that expresses the modulation in multiple values is realized in principle by controlling a continuous change in the reflected light intensity (modulation) in a finite number of discrete levels. It is a natural course of events to apply to the multilevel recording the phase change medium that performs information read and write operations by using a chance in the reflected light intensity.
However, no recording medium is currently available that takes advantage of the phase change recording to realize the capability of actually performing such a recording in multilevel levels, or preferably overwriting repetitively. This is because both the phase change medium and the recording method that record data at a plurality of modulation levels with good reproducibility are still in the development stage. Typically, the recording levels are two states-crystal and amorphous states-or three states at most (JP-A 61-3324, 62-259229 and 10-124925).
There is also an effort to control an average optical characteristic in multiple levels by changing a mixture ratio of different crystalline states or of crystalline and amorphous states.
However, an optical characteristic difference among different crystalline states is too small to identify and it is difficult to control the mixture ratio of crystalline and amorphous states in multiple levels with good reproducibility. Obtaining the four or more levels with good reproducibility is not easy. Such a mixed state is unstable and the amorphous portion easily transforms into crystal, gibing rise to a problem of poor stability of recorded information over time.
The problems described above can be solved by causing a recrystallization to occur in the recording layer during the solidifying of she recording layer melted by the recording beam and by using the recrystallization in controlling the size of the amorphous mark in multiple levels.
In summary, this invention includes the following inventions.
(1) A multilevel recording/reproducing method comprising the steps of: radiating a recording energy beam against an information recording medium having a recording layer to locally melt the recording layer, the recording layer being adapted to produce a phase change between a crystalline state and an amorphous state upon being radiated with an energy beam; and forming an amorphous mark by cooling during a solidifying process to record information in the medium; wherein the size of the amorphous mark is controlled mainly by a competition between a recrystallization process and an amorphization process during the solidifying process; wherein an intensity of reflected light from a reproducing light beam radiated region is controlled in three or more multiple recording levels according to an optical characteristic difference between a crystalline region and a amorphous region and their areas.
(2) A method according to item (1), wherein a recording energy beam is radiated onto a region formed with the amorphous mark to melt the recording layer and thereby erase the amorphous mark and, during the solidifying process, an amorphous region and a recrystallized region are newly formed to overwrite the amorphous mark.
(3) A method according to item (1) or (2), wherein the recording energy beam and the reproducing energy beam have spot diameters on a recording layer surface of 2 xcexcm or less.
(4) A method according to item (3), wherein the recording and reproducing light beams have elliptical spots on the recording layer surface with their major axes oriented in a direction substantially perpendicular to the direction of beam scan.
(5) A method according to any one of items (1) to (4), wherein when the recording energy beam is scanned relative to the recording medium to form melted regions to form amorphous marks along the scanning direction, the size of the amorphous mark is controlled by changing a width, with respect to the scanning direction, of the amorphous mark and the width of the amorphous mark is made smaller than the width, with respect to the scanning direction, of the reproducing energy beam at any of the multiple recording levels.
(6) A method according to any one of items (1) to (5), wherein when the recording energy beam is scanned relative to the recording medium to form melted regions to form amorphous marks along the scanning direction, the size of the amorphous mark is controlled by changing a length, with respect to the scanning direction, of the amorphous mark and the length of the amorphous mark is made smaller than the length, with respect to the scanning direction, of the reproducing energy beam at any of the multiple recording levels.
(7) A method according to any one of items (1) to (6), wherein when a transition is made from one recording level section to another, the transition passes, without fail, through a recording level section that corresponds to a crystalline state.
(8) A method according to item (7), wherein the amorphous marks are isolated, surrounded by a crystalline region and intervals between reflected light intensity peaks corresponding to the isolated amorphous marks are made constant at a reference length T.
(9) A method according to item (8), wherein the interval between isolated reflected light intensity peaks is an integer times the reference length T, and a multilevel recording is performed by using two variables consisting of a peak-to-peak interval LT (L is n kinds of integers) and an m-step recording level.
(10) A method according to item (7), wherein the recording label section has a trapezoidal waveform with n kinds of lengths, and at least one of the length of the trapezoidal section and an interval between the trapezoidal sections is modulated.
(11) A method according to any one of items (1) to (6) wherein when a transition is made from one recording level section to another, the transition is made continuously without passing through the reference recording level.
(12) A method according to any one of items (1) to (11), wherein a part or all of a recording energy beam radiation time in one recording level section is divided into one or more recording pulse sections and one or more interrupt sections, the power of the recording energy beam in the recording level section is set to a power Pw strong enough to melt the recording layer during the recording pulse section and to a power Pb, including 0, smaller than Pw during the interrupt section, and the size of the amorphous mark is controlled by changing a radiation pattern of the recording energy beam in the radiation time.
(13) A method according to item (12), wherein the power Pb of the recording energy beam radiated during the interrupt section meets a condition of 0 less than Pb less than 0.2 Pw.
(14) A method according to item (12) or (13), wherein the radiation pattern of the recording energy beam in the radiation time is changed by changing the magnitudes of Pb and Pw.
(15) A method according to any one of items (1) to (14), wherein the radiation pattern of the recording energy beam in the radiation time Is changed by changing the lengths of the recording pulse section and/or the interrupt section.
(16) A method according to any one of items (1) to (16), wherein a diameter rb of the reproducing light beam is set equal to a spatial length Ts of the recording section or more.
(17) A method according to item (16), wherein a part or all of a recording energy beam radiation time for forming one recording level section is divided into one recording pulse section and one interrupt section accompanying the recording pulse section before or after it, the power of the recording energy beam in the recording level section is set to a power Pw strong enough to melt the recording layer during the recording pulse section and to a power Pb, including 0, smaller than Pw during the interrupt section, and the size of the amorphous mark is controlled by changing Pw, Pb, the recording pulse section length and/or the interrupt section length.
(18) A method according to item (17), wherein a length of the recording level section is constant at a reference length T and the size of the amorphous mark is controlled by changing a duty ratio of the recording pulse section to the recording level section.
(19) A method according to any one of items (1) to (18), wherein the number of recording levels is four or more.
(20) A method according to any one of items (1) to (19), wherein a reflected light intensity range including a strongest reflected light intensity Rc and a weakest reflected light intensity Ra is divided into m sub-ranges (m greater than 1), the m sub-ranges are set so that a sub-range having a maximum reflected light intensity includes the strongest reflected light intensity Rc and a sub-range having a minimum reflected light intensity includes the weakest reflected light intensity Ra, and which level a reflected light intensity obtained corresponds to is determined by checking which of the m sub-ranges the reflected light intensity belongs to.
(21) A method according to item (20), wherein the m sub-ranges have equal magnitudes to each other in the reflected light intensity.
(22) A method according to item (20), wherein the magnitude of each of them sub-ranges increases as the sub-range comes closer to Rc.
(23) A multilevel recording medium having a recording layer, wherein the recording layer changes its phase between a crystalline state and an amorphous state upon being radiated with an energy beam, and a recrystallization from a melted state in the recording layer proceeds substantially by a crystalline growth from a crystalline region.
(24) A medium according to item (23), wherein the recording layer has an alloy composition containing Sb.
(25) A medium according to item (24), having an SbTe alloy composition containing Sb in excess of an eutectic point.
(26) A medium according to item (24), wherein the recording layer includes the following composition
Mx(SbyTe1xe2x88x92y)1xe2x88x92x 
where 0 less than xxe2x89xa60.2, 0.6xe2x89xa6y, and M is at least one element selected from the group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Bi, Cr, Co, O, S, N, Se, Ta, Nb, V, Zr, Hf and rare earth metals.
(27) A medium according to item (26), wherein the recording layer includes the following composition
Mxe2x80x2xcex1Gexcex2(Sbxcex3Te1xe2x88x92xcex3)1xe2x88x92xcex1xe2x88x92xcex2
where Mxe2x80x2 is In and/or Ga, 0.001xe2x89xa6xcex1xe2x89xa60.1, 0.001xe2x89xa6xcex2xe2x89xa60.15, and 0.65xe2x89xa6xcex3xe2x89xa60.85.
(28) A medium according to any one of item (23) to (27), wherein protective layers are provided over and under the recording layer, and a reflection layer is provided over that surface of one of the protective layers which is on the opposite side of the recording layer.
(29) A medium according to item (28), wherein the recording layer has a thickness of 1 nm to 30 nm, the dielectric protective layer provided between the recording layer and the reflection layer has a thickness of 60 nm or less, and the reflection layer is an alloy comprising Al, Ag or Au mainly.
(30) A medium according to item (28) or (29), wherein the reflection layer has a sheet resistivity of 0.1 to 0.6 xcexa9/xe2x96xa1.
(31) A medium according to any one of items (23) to (30), used in the method of item (18), wherein the reflected light intensity exhibits a substantially linear change responsive to a change in the duty ratio used.
(32) A medium according to anyone of items (23) to (31), used in the method of item (18), wherein when the duty ratio is 95% or more, the amorphous mark is not formed.
(33) A medium according to item (31) or (32), wherein when the duty ratio at which the minimum reflected light intensity Ra is obtained is Da (%) and the duty ration at which the maximum reflected light intensity Rc is obtained is Dc (%), then Dcxe2x88x92Da greater than 50%